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    <script>
        /* 
             九九乘法表:双重for循环

             *
             **
             ***
             ****

             i    num(j 的最大值)
             0     1
             1     2
             2     3
             
             i + 1 = num
        */
        let num1 = prompt('请输入行数');
        for (let i = 1; i <= num; i++) {
            let star = '';
            for (let j = 1; j <= i; j++) {
                star = star + (i + '*' + j + '=' + i * j) + '   ';
            }
            console.log(star);
        }
        let num = prompt('请输入行数');
        for (let i = num; i > 0; i--) {
            let star = '';
            for (let j = 1; j <= i; j++) {
                star = star + (i + '*' + j + '=' + i * j) + '   ';
            }
            console.log(star);
        }
    </script>
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